AT A GLANCE
- Common Name: Great Raft Spider
- Scientific Name: Dolomedes plantarius
- Category: Invertebrates
- Subcategory: Arachnids
- Range: Temperate Europe and Western Asia, with isolated, highly fragmented populations across the British Isles, France, the Low Countries, Scandinavia, Central Europe, and east to Russia
- Habitat Type: Pristine lowland wetland fens, grazing marsh drainage ditches, peat bogs, and undisturbed lake margins with dense emergent water-soldier (Stratiotes aloides) plants
- IUCN Red List Status: Vulnerable
- Lifespan: 2 to 3 years
- Diet: Carnivorous/Semi-aquatic predator (water boatmen, pond skaters, dragonfly nymphs, damselflies, tadpoles, and even small fish like sticklebacks)
- Average Size/Weight: Female body length 18 to 23 mm (leg span up to 70 mm); male body length 10 to 15 mm; one of Europe’s largest native spiders
Overview
The Great Raft Spider (Dolomedes plantarius), often known as the Fen Raft Spider, is one of the largest, rarest, and most magnificent arachnids in Europe. Inhabiting pristine, undisturbed lowland fens and wetland ditch systems, this giant semi-aquatic hunter does not build a capture web. Instead, it uses the surface tension of water as its hunting ground, resting its front legs on the surface to detect ripples caused by aquatic insects, tadpoles, and small fish before dashing across the water to deliver a venomous strike.
Biology and Physical Characteristics
Dolomedes plantarius is a massive hunting spider in the nursery web family Pisauridae, with mature females achieving a body length of over 2 cm and a formidable leg span exceeding 7 cm. Its robust body is covered in a dense coat of velvety, water-repellent (hydrophobic) hairs that trap a thin layer of air around the spider when submerged, allowing it to breathe underwater. The coloration is a deep, rich chocolate-brown or bronze-black, bordered along each side of the cephalothorax and abdomen by an immaculate, glowing white or cream longitudinal stripe. The legs are thick and powerful, armed with fine sensory hairs (trichobothria) that detect infinitesimal water surface vibrations.
Behavior and Ecology
When hunting, the Great Raft Spider sits with its rear legs anchored on emergent aquatic vegetation (such as water-soldier, Stratiotes aloides) and its front legs resting lightly on the surface tension of the water. Highly sensitive to water vibrations, it can distinguish between the random ripples of falling windblown leaves and the rhythmic thrashing of a struggling insect or tadpole. When prey is detected, it gallops across the water surface with remarkable speed, delivers a quick bite, and pulls the prey ashore to feed. If alarmed by a bird, it can dive beneath the surface and cling to submerged plants for up to an hour. In late summer, females spin gigantic silk ‘nursery webs’ the size of a tennis ball suspended in emergent marsh plants to shelter hundreds of newly hatched spiderlings.
Conservation and Human Impact
The Great Raft Spider is classified as Vulnerable globally on the IUCN Red List and is strictly protected under European wildlife legislation. Historically pushed to the brink of extinction across Western Europe due to the extensive drainage of lowland fens for agriculture and the degradation of ditch water quality by agricultural fertilizer runoff, it has been the subject of successful captive-breeding and translocation programs in the UK and Netherlands.
Interesting Factoid
The Great Raft Spider can walk, run, and hunt on water like a pond skater, and when threatened, it dives underwater and stays submerged for nearly an hour, breathing from a silvery bubble of air trapped in its velvety fur.